Metagratings for Efficient Wavefront Manipulation

Abstract

Recently, it was revealed that conventional gradient metasurfaces are fundamentally limited on their overall efficiency to reroute the impinging waves towards arbitrary directions in reflection and transmission. Their efficiency is particularly limited for extreme wavefront transformations. In addition, due to the fastly varying impedance profiles that these surfaces require, they usually need high-resolution fabrication processes, limiting their applicability and overall bandwidth of operation. To address these issues, the concept of metagrating was recently introduced, enabling engineered surfaces capable of manipulating light with unitary efficiency even in the limit of extreme wavefront manipulation. Metagratings are periodic or locally periodic arrays operated in the few-diffraction order regime. Their unit cell contains carefully designed scatterers that, in contrast with conventional metasurfaces, do not need to support a continuous gradient of surface impedance, and consequently are far simpler to fabricate and can afford broader bandwidths because of the larger footprint of the constituent elements. Tailoring the coupling towards the few available diffraction channels, metagratings enable wavefront transformations with high efficiency. In this paper, we review the physical mechanisms behind the functionality of metagratings and provide an overview and outlook of the recent progress in this field of science and technology.

Extreme Diffraction Management in Phase‐Corrected Gradient Metasurface by Fourier Harmonic Component Engineering

“…[28,29] Metagratings (MGs), constituted of periodic, typically sparse and polarizable particles, constitute another promising way to enable wavefront transformations with high efficiency. [30][31][32] Different from PGMs, MGs align the propagating Floquet mode with desired harmonic components instead of locally superposing a transverse momentum. For example, single-order deflection can be achieved by periodic composition of electrically or magnetically polarizable particles to eliminate the undesired refraction.…”

Microacoustic Metagratings at Ultra‐High Frequencies Fabricated by Two‐Photon Lithography

“…Acoustic metagratings [38] are periodic arrays of discrete metaatoms, which can diffract the incident acoustic energy via multiple diffraction orders as shown in Figure 1a. The number of modes n and associated diffraction angles 𝜃 n are determined by [20,21,[23][24][25][26][27][34][35][36][37] the acoustic wavelength 𝜆 and the metagrating lattice constant d, based on Bragg's condition, that is, d = n𝜆/sin 𝜃 n .…”

High-efficiency broadband blazed metagrating working in visible light

“…The realization of efficient * Author to whom any correspondence should be addressed. directional diffraction has many applications in the spectral analysis [4][5][6], holographic display [7][8][9], imaging [10,11], mode selectivity and coupling [12,13], and other fields. The conventional way to effectively direct energy to high diffraction orders is completed by a blazed grating with triangular grooves, working under the Littrow configuration [14].…”

“…There has been much work to use the metasurface to achieve beam deflection, directional radiation, and many other applications [19][20][21]. Nonetheless, metasurface is challenging to achieve high-efficiency directional radiation by structures with discretized phases [11,22]. In addition, the structure is more complex than the conventional gratings and challenging to fabricate.…”

“…In addition to the above two methods, Ra'di et al [22] proposed the concept of metagratings, which enables unitary efficiency without the need for deeply subwavelength elements, turning away from the discretization of a continuous phase or impedance profile. Metagratings are periodic or locally periodic arrays working in the few-diffraction order (FDO) regime [11,23]. Different from metasurfaces, metagratings are set the periodically to align the direction of one of the propagating higher-order Floquet modes with the desired direction of reflection, which engineers scattered in each unit cell so that its radiation field has nulls in the direction of all propagating Floquet modes other than the one that is aligned with the desired direction of reflection [24].…”